If smallblacksun wanted to refer only to stereotypes about gender roles he/she should have said 'those stereotypes exist for a reason.' Instead he/she said 'stereotypes exist for a reason,' which is a much broader statement.
>Because it's [stereotyping is] easier than critical thinking?
In a way, yes.
In fact stereotyping is like writing a script rather than repeating the same process over and over. You may need to amend the script for a given situation of course, but it serves as a useful first approximation of a solution.
It is critical thinking that leads one to effective use of stereotypes: it's not always raining when it's cloudy outside but it's a useful indicator.